1. Field of the Invention
The invention relates to an external-cavity laser and in particular to an external cavity tuneable laser that is especially adapted as optical transmitter for wavelength-division multiplexed optical communication networks.
2. Description of the Related Art
The use of lasers as tuneable light source can greatly improve the reconfigurability of wavelength-division multiplexed (WDM) systems or of the newly evolved dense WDM (DWDM) systems. For example, different channels can be assigned to a node by simply tuning the wavelength. Also, tuneable lasers can be used to form virtual private networks based on wavelength routing, i.e., photonic networks.
Different approaches can be used to provide tuneable lasers, such as distributed Bragg reflector lasers, VCSEL lasers with a mobile top mirror, or external-cavity diode lasers. External-cavity tuneable lasers offer several advantages, such as high output power, wide tuning range, good side mode suppression and narrow linewidth. Various laser tuning mechanisms have been developed to provide external-cavity wavelength selection, such as mechanically adjustable or electrically activated channel selector elements.
U.S. Pat. No. 6,526,071 describes an external-cavity tuneable laser that can be employed in telecom applications to generate the centre wavelengths for any channel on the International Telecommunications Union (ITU) grid. The disclosed tuneable laser includes a gain medium, a grid generator and a channel selector, both grid generator and channel selector being located in the optical path of the beam. The grid generator selects periodic longitudinal modes of the cavity at intervals corresponding to the channel spacing and rejects neighbouring modes. The channel selector selects a channel within the wavelength grid and rejects other channels. The grid generator is dimensioned to have a free spectral range (FSR) corresponding to the spacing between gridlines of a selected wavelength grid (an ITU grid) and the channel selector is dimensioned to have a FSR broader than that of the grid generator which is itself broader than the FSR of the cavity.
Typically, the grid generator is a Fabry-Perot etalon defining a plurality of transmission peaks (also referred to as passbands) defining periodic longitudinal modes. To select a periodic longitudinal mode (i.e., a lasing channel on the ITU grid), several channel selecting mechanisms have been considered, including rotating a diffraction grating, mechanically translating a wedge-shaped etalon, or varying the voltage or current supplied to an electro-optically controlled element.
J. De Merlier et al. in “Full C-Band External Cavity Wavelength Tunable Laser Using a Liquid-Crystal-Based Tunable Mirror”, published in IEEE Photonics technology Letters, vol. 17, No. 3 (2005), pages 681-683, disclose an external cavity tuneable laser containing a fixed etalon with a FSR of 50 GHz and a liquid crystal (LC) based tuneable mirror. The tuneable mirror is an optical resonator that works in reflection, exhibiting one reflection peak over a wide wavelength range which can be tuned over the whole C-band by adjusting the amplitude of the ac voltage signal. The laser consists of a chip containing a gain and a phase section. The integration of the phase control on the chip avoids the need for mechanical tuning of the cavity length.
An external cavity tuneable laser with an etalon as grid generator and an LC-based tuneable mirror is described in WO patent application No. 2005/041371.
In order to accommodate increasing optical communication traffic, DWDM systems with channel spacing of 50 GHz and even of 25 GHz have been recently developed. As DWDM uses narrower channel spacing, wavelength (frequency) accuracy of transmitter lasers over the entire tuning (e.g., the C-band) and operating temperature range has become an important issue. DWDM systems with 50 GHz channel spacing typically require an accuracy of ±2.5 GHz about the lasing frequency, whereas systems with 25 GHz generally require a frequency accuracy of ±1.25 GHz.
As tuneable elements are configured for narrower channel separation, decreasing component tolerances and thermal fluctuations become increasingly important. Spatial misalignments of optical components of the laser device may arise from temperature variations due to expansions and contractions associated to the various components, which will reduce wavelength stability and generally reduce the performance of the laser. The laser response needs to be stabilised across a relatively wide temperature range, typically ranging from −5° C. to 70° C. To ensure thermal stability, many telecommunication laser devices are mounted on a common platform, which exhibits high thermal conductivity and is subject to the thermal control of one or more thermo-electric coolers (TECs). Temperature control allows for maintenance of thermal alignment of the optical components.
In an external cavity laser, a resonant external cavity is formed with optical path length Lopt between a first mirror, typically the reflective rear surface of the gain medium, and a second mirror, the end mirror. The free spectral range (FSR) of the laser cavity, i.e., the spacing between the cavity modes, depends on the optical path length, owing to the relation
                              (                      F            ⁢                                                  ⁢            S            ⁢                                                  ⁢            R                    )                =                              c            0                                2            ⁢                          L              opt                                                          (        1        )            wherein c0 is the speed of light in vacuo.
The optical path length of an external cavity laser is a sum of the products of indices of refraction and optical thicknesses of the various elements or components present in the optical path across the external cavity, including the air present within the cavity. Thus, the optical path length of the laser cavity can be shown asLopt=ΣiniLi  (2)where ni (i=1, . . . , m) is the refractive index of the medium filling the ith-optical element (component) that the light encounters in the cavity and of the cavity itself (i.e., the free space, nFS≈1), while Li is the thickness of the ith-element and the physical length the light travels in free space (i.e., the free-space physical length). The external cavity can be thought of as an optical resonator composed of two confronting and reflective, generally parallel, surfaces separated by a length, which is defined as the physical length of the cavity, L0. In general, Lopt≧L0.
U.S. Pat. No. 6,658,031 discloses a laser apparatus that uses an active thermal adjustment of a laser cavity reflective element to minimise losses and provide wavelength stability. A compensating member is coupled to a reflector and configured to thermally position the one reflector with respect to the other reflector in order to maintain an optical path length that does not vary with temperature (except during active thermal control of the compensating member). The thermal positioning may be carried out by a thermoelectric controller operatively coupled to the compensating member and configured to thermally adjust the compensating member by heating or cooling thereof.
In U.S. Pat. No. 6,724,797, an external-cavity laser device is disclosed, wherein selective thermal control is applied to optical components having a high susceptibility to thermal misalignments. The gain medium and the optical output assembly, which are temperature sensitive components, are mounted on a thermally conductive substrate. A TEC is coupled to the substrate to allow for the gain medium and the output assembly to be thermally controlled independently from the end mirror and other components of the external cavity laser. Components of the external cavity, which are thermally isolated from the thermally conductive substrate, may comprise a channel selector and a tuning assembly.
From Eq. (2) it can be seen that Lopt may be adjusted by physical adjustment of the spacing between the two end mirrors defining the external cavity and/or by adjusting the refractive index of the material present in the external cavity. Semiconductor gain media such as InGaAs and InGaAsP have generally high refraction indices that exhibit relatively large variations with temperature and therefore can significantly contribute to the overall external cavity optical path length.
U.S. Pat. No. 6,763,047 describes an external cavity laser apparatus that uses an active thermal adjustment of the external cavity to minimise losses and provide wavelength stability. The apparatus of the cited patent includes a thermally conductive platform, a gain medium and an end mirror thermally coupled to the platform and a thermoelectric controller thermally coupled to the platform and configured to cause the platform to expand and contract in response to a temperature change of the platform, thereby adjusting the optical path length of the cavity. Heating or cooling of the platform by the thermoelectric controller provides temperature control of the gain medium refractive index via thermal conduction with gain medium and/or thermal expansion or contraction of the platform to control the spacing between the end mirrors. A control element is operatively coupled to the thermoelectric controller to provide control instructions regarding heating or cooling of the platform, and hence of the gain medium.
In the field of integrated circuits, different techniques and structures have been proposed to remove the heat generated by the operation of semiconductor devices in order to maintain the temperature of the devices within a predetermined range.
U.S. Pat. No. 4,442,450 describes an electronic semiconductor package having a support substrate, an integrated circuit semiconductor device mounted on said substrate, a cover mounted on said substrate disposed over said device and thermal bridge for conducting heat from said device to said cover. The thermal bridge comprises a relatively thick metal sheet provided with grooves and cuts that make the thermal bridge bendable, said metal sheet being overlaid by a spring element to selectively urge part of the bridge into contact with the device.
In U.S. Pat. No. 4,479,140, a thermal bridge element is used in a semiconductor package to conduct heat from a semiconductor device mounted on a substrate to a cold plate or cap in close proximity to the device.